Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. Numerous photosensitive printing plate formulations have been developed to meet the demand for fast, inexpensive processing and long press runs.
Photosensitive printing elements generally comprise a support layer, one or more photosensitive layers, an optional slip film release layer, and an optional protective cover sheet. The protective cover sheet is formed from plastic or any other removable material that can protect the plate or photocurable element from damage until it is ready for use. The slip film may be disposed between the protective cover sheet and the photocurable layer(s) to protect the plate from contamination, increase ease of handling, and act as an ink-accepting layer.
Flexographic printing plates desirably work under a wide range of conditions. For example, they should be able to impart their relief image to a wide range of substrates, including cardboard, coated paper, newspaper, calendared paper, and polymeric films such as polypropylene and the like. Importantly, the image should be transferred quickly and with fidelity, for as many prints as the printer desires to make.
To prepare a printing plate with typical commercially available equipment, an image-bearing transparency or negative, i.e., a transparent film having opaque regions corresponding to the reverse of the image which one desires to impart to a printing plate, is placed on a glass platen, and covered with a transparent, polymeric cover film. The transparency and cover film are secured by vacuum to the platen, and a layer of photopolymerizable resin laminated to a backing sheet is placed on the cover film. Actinic radiation is shined through the glass platen toward the backing sheet. The regions of the resin which are impinged by the actinic radiation undergo polymerization or “curing” to form a cured resin that is insoluble in a solvent used for washing away uncured regions of the photopolymer to “develop” or reveal the relief image in the cured resin.
The regions of the resin layer which were protected from the actinic radiation by the opaque regions of the transparency are washed away using a developer solution. The cured regions are insoluble in the developer solution, and so after development a relief image formed of cured photopolymerizable resin is obtained. The cured resin is likewise insoluble in certain inks, and thus may be used in flexographic printing. The liquid photopolymerizable resin may also be exposed to actinic radiation from both sides of the resin layer.
Compressible materials have found great utility in flexographic printing, particularly in the printing of corrugated stock or similar substrates that have an uneven, deformable surface. With such substrates, the printing plate must be flexible so that it will conform to the uneven surface and evenly deliver a coating of ink thereon. However, if the plate is too soft or flexible, the image on the plate will distort under the pressure used to contact the plate with the substrate, and thus will not transfer the image with the desired fidelity.
Compressible printing plates are also useful in:                A. printing on wide webs (films of cellophane, polyethylene, polypropylene, polyester, vinyl and paper having a web thickness in the range of about 0.5 to 5 mils to overcome sag and distortion and machine gauge variation by the compressibility of foam.        B. printing safety papers (checks, bond and stock forms) with even uniform constant tone in background print.        C. process printing (4 color printing) where uniformity of ink laydown is critical to proper color development.        
Compressible printing plates also have a longer life. The mechanical shock to the plate, in each print motion, causes a gradual wearing of the relief, gradually leading to loss of sharpness in print. The compressible layer absorbs the mechanical shock leaving the relief printing surface relatively unaffected (minimal flattening or distortion) resulting in longer plate life.
Mounting a printing plate onto a compressible layer allows for that layer to deform and compress with the substrate, while the printing plate can then be made to withstand the rigors of direct contact with the substrate. The alternate approach involves making the printing plate softer, which can lead to an undesirable growth of the characters under the required printing impression pressure, particularly when printing on rough or uneven stock or on presses with uneven impression and/or plate cylinders.
On their own, high durometer plates can often damage the deformable substrate during the printing process. However, a higher durometer plate, in combination with a base compressible layer, can potentially solve both of these issues. In this way, one can use a fairly hard, i.e. high durometer, plate which will not provide a distorted image, and take advantage of the compressibility of the foam backing to allow the plate to bend and flex, and thereby contact all regions of an uneven substrate. Typical compressible layers consist of foamed materials, oftentimes polyurethane or other thermoplastic materials, and are often laminated to the back of a cured image-bearing printing plate using tape or other pressure sensitive adhesives.
A persistent problem with this approach however is that it is very difficult to secure the foam materials to the back of the plate. It is difficult to apply the adhesive uniformly, and foam materials exhibit the problem that they stretch during mounting to the plate and, if stabilized, cause buckling when the plate is flexed. In addition, these steps are time consuming, and therefore can be quite costly to the trade shops, mounters, and printers who are getting the plate ready for press. The extra backing layer can also actually interfere with the true compressibility of the plate construction.
In most cases, the printing plate is already completely exposed and developed before the compressible layer is secured thereto, although several examples exist wherein liquid or uncured solid photopolymeric materials are cast and directly exposed on top of the compressible layer. However, in all of these processes, the compressible layer is still formed in a separate step and then pretreated with an adhesive or some sort of tie coat layer to laminate the compressible layer to the printing plate.
U.S. Pat. No. 5,325,776, to Rather, Sr. et al., describes the use of polyurethane foam materials as compressible layers to be used in concert with a flexographic printing plate. However, the polyurethane foam is not UV-curable and requires a coating of an adhesive layer on top of the foam to secure the foam to the printing plate.
U.S. Pat. No. 5,894,799 to Bart et al., describes the use of an elastomeric photopolymer as the compressible layer, wherein the elastomeric photopolymer contains open cells on the surface to create the compressible nature. Photopolymerization is used only to create the open cells, and an adhesive layer is still required in the plate-laminating step. The compressible layer must then be pre-exposed, developed, and post-cured prior to being adhered to the printing plate. In addition, the elastomeric photopolymer of the compressible layer is a styrenic block copolymer-based photopolymer, not a polyurethane, and thus requires solvent development.
U.S. Pat. No. 5,962,111 to Rach, describes the casting of a liquid photopolymer layer onto a compressible material with open cells, and then insufficiently curing that layer to allow it to serve as a tie coat for the printing plate layer. A second coating of liquid resin is then coated on top of the pre-cured layer, and the plate is exposed and developed. In this instance, the compressible layer is required to have open cells, and is pre-treated with the resin of choice for making the printing plate. In essence, the photopolymer is serving as its own adhesive. Again, this system requires multiple steps until the final system is in place.
As is readily seen, while various methods have been suggested in the prior art for preparing compressible flexographic printing plates, there remains a need in the art for a simple manufacturing process for preparing compressible photopolymer plates that overcomes the shortcomings of plates made according to prior art methodologies.
The purpose of the present invention is to provide a one-step system for the production of compressible printing plates using conventional plate making methods and materials. Utilizing a photocurable resin, such as a polyurethane (meth)acrylate resin that contains thermoplastic microspheres, a compressible foam can be created and cast along with the first photocurable layer in which the relief image is formed and two layers cured simultaneously to form the compressible plate system of the invention. The one-step process of the invention eliminates the need for adhesives to lamination the compressible layer to the back of the exposed printing plate.
Furthermore, while most urethane foams are actually created during the polymerization step or by post-curing/blowing of the urethane, the present invention does not require any alteration of the urethane, only a blend of the final resin with the microspheres.
U.S. Pat. No. 6,287,638 to Castelli et al., describes the use of thermoplastic microspheres in the formation of printing blankets. The microspheres are dispersed into a thermoplastic medium and then vulcanized using heat to form a crosslinked matrix. The surface of the blanket is then cast on top of the surface of a carcass material. However, this is a non-ultraviolet curing system and is also a multi-step system, require a separate compressible layer-formation step before the printing element is ready to be used. Adhesion in these systems often requires the use of a tie coat layer to affix the printing layer on top of the compressible layer. Often times, the microsphere layer is covered with layers of fabric, so that the printing layer does not come into contact with the compressible layer per se, but rather comes into direct contact only with the fabric.